A Coriolis mass flow meter is a meter for measuring liquid mass flow and other information (including, but not limited to, density, temperature, etc.) by means of Coriolis effect. Such a flow meter usually includes one or more straight or curved flow tubes. The liquid to be measured flows into the flow tube from one end thereof and out from the other end thereof. A commonly seen dual-flow-tube Coriolis mass flow meter 100 as shown in FIG. 1 is taken as an example, which structure is usually as follows: two flow tubes 111, 112 are parallel to each other and curved into U-shape; inlet ends of the flow tubes 111, 112 are fixed to an inlet manifold block 121, outlet ends of the flow tubes 111, 112 are fixed to an outlet manifold block 122, the inlet manifold block 121 is connected with an inlet flange 131, the outlet manifold block 122 is connected with an outlet flange 132, and there is usually a transverse tube 123 for support of and connection between opposing sides of the two manifold blocks 121 and 122; liquid to be measured flows into the inlet manifold block 121 through the inlet flange 131; the inlet manifold block 121 divides the liquid stream into two substantially equal liquid streams that flow into two flow tubes 111, 112 respectively; the separated liquid flows via the flow tubes 111, 112 into the outlet manifold block 122; the outlet manifold block 122 collects two liquid streams into one stream which flows via the outlet flange 132 into the conduit; a vibration exciter 141 is arranged in the middle portion of the flow tubes 111, 112, and a vibration sensor 142 is arranged at both sides of the vibration exciter 141 at a distance therefrom. The portion between the inlet flange 131 and the outlet flange 132 is a core sensing portion of the Coriolis mass flow meter. The portion of the flow tubes between two vibration sensors 142 is commonly named as a measuring section. The vibration sensors 142 are connected by cables with a signal processing device and a display device (not shown). The above all forms the Coriolis mass flow meter. The overall structure of other types, such as a single-flow-tube and a multi-flow-tube Coriolis mass flow meter, is also well-known to those skilled in the art and will not be described herein.
It is a disclosed technique that the Coriolis mass flow meter is used for measuring information of liquid, such as mass and density, which has been detailed in e.g. U.S. Pat. No. 4,491,025 published on Jan. 1, 1985. As known by those skilled in the art, the flow tube of the Coriolis mass flow meter vibrates in an inherent mode when in operation, thereby generating a corresponding resonance frequency. This resonance frequency is directly associated with flow tubes and the liquid therein. When being excited, the flow tubes will vibrate at a substantially fixed frequency. When there is no liquid in the flow tube or the liquid therein does not flow, the phases at the various points of the flow tube are the same (the phase mentioned herein means that the flow tube vibrates substantially in a path of sine signals, which is indicated by the formula f(t)=A sin(ωt+φ), wherein φ is the phase). When the liquid in the flow tube flows, a Coriolis acceleration is generated on the flow tube due to the presence of the Coriolis effect. This results in different phases at various points of the flow tube, wherein the phase at the inlet side lags behind that of the vibration exciter, and the phase at the outlet side exceeds that of the vibration exciter. Vibration sensors, which are respectively located at the inlet side and the outlet side with respect to the vibration exciter, measure the motion of the flow tubes. There is a particular relationship between the phase differences measured by the vibration sensors and the mass flow of the liquid flowing through the measuring section. The mass flow of the liquid can be measured by a phase difference of signals of the vibration sensors.
A vibrating tube density meter is substantially the same as the mass flow meter in terms of structure, and usually made of one or more straight or curved flow tubes fixed between the inlet manifold block and the outlet manifold block. The difference therebetween is that the vibrating tube density meter measures density in accordance with the relationship between the measured liquid density and the intrinsic frequency when the liquid passes through the flow tube, whereas the mass flow meter measures the mass flow of the liquid in accordance with the phase difference at different positions of the flow tube. The flow tube of the vibrating tube density meter vibrates in an inherent mode when in operation, thereby generating a corresponding resonance frequency. Change in density of the liquid in the flow tube leads to a variation of resonance frequency of the flow tube. The density of the liquid in the flow tube can be obtained by measuring the resonance frequency of the flow tube.
Generally speaking, the Coriolis mass flow meter and the vibrating tube density meter are fixed with vibrating sheets (also known as damping plates or bracing bars), each of which is disposed at a position with a certain distance from the inlet or outlet of the flow tubes, for instance, vibrating sheets 150 as shown in FIG. 1. Conventional vibrating sheets are usually thin flat sheets with through holes. For instance, in the dual-flow-tube Coriolis mass flow meter 100 as shown in FIG. 1, the prior art vibrating sheet 150 usually has two through holes, each of which has an entire circumference, and two flow tubes 111, 112 extend through the two through holes and are fixedly welded to the vibrating sheets 150 along the entire circumferences of the through holes by fusion welding or brazing. The desired vibrating mode of the Coriolis mass flow meter 100 when excited is that the two flow tubes 111, 112 move towards or apart from each other simultaneously, and then the phases of the movements of the two flow tubes are opposite to each other, which is called “out-of-phase” vibration, as shown by arrows A and A′ in FIG. 2. The two flow tubes may also vibrate towards the same direction at the same time, which is called “in-phase” vibration, as shown by arrows B and B′ in FIG. 3. When the flow tube is interfered by external vibration, the in-phase vibration and the out-of-phase vibration will be superposed together, which can influence the precision of flow measurement. One of the important reasons for superposition of the in-phase vibration and the out-of-phase vibration is that the in-phase vibration and the out-of-phase vibration have an identical revolving axis. When there is no vibrating sheet, the flow tube is directly fixed to the manifold blocks, and revolving axes of the in-phase vibration and the out-of-phase vibration are at a position near the junction of the flow tube and the manifold block. The in-phase vibration and the out-of-phase vibration have similar vibration frequencies. The more similar the frequencies are, the greater the interference with the out-of-phase vibration by the in-phase vibration. One way to avoid such case is to add a vibrating sheet at a position with a distance from the junction of the flow tube and the manifold block so as to connect the two flow tubes to each other. Then, the revolving axes N, N′ of the in-phase vibration are adjacent to the junctions 115 of the flow tubes and the manifold blocks, and the revolving axes M, M′ of the out-of-phase vibration are adjacent to the welded junctions 116 of the flow tubes and the vibrating sheets. The in-phase vibration and the out-of-phase vibration have not only different revolving axes but also different vibration frequencies. The in-phase vibration can hardly interfere with the out-of-phase vibration, which is good for accurately measuring desired data by the Coriolis mass flow meter and the vibrating tube density meter.
There are various types of vibrating sheets in the prior art. For instance, an international patent application No. WO95/03529 published on Feb. 2, 1995 describes several vibrating sheets as shown in FIGS. 4 to 6, wherein FIG. 4 shows a common type of vibrating sheet, and FIGS. 5 and 6 show two vibrating sheets aiming to reduce stress concentration as taught by the above-mentioned patent application. In the technical solution thereof, the flow tubes are fixed with the vibrating sheet by using a zinc-copper alloy to weld along the entire circumferences of the through holes after the two flow tubes pass through the corresponding through holes, as shown in FIG. 7.
The Chinese patent application No. CN101745721A published on Jun. 23, 2010 provides another type of vibrating sheet, as shown in FIG. 8, characterized in that a recess is disposed on the surface of the vibrating sheet around the through hole which the flow tube passes through, mainly for the purpose of facilitating manual argon arc welding. During fixing operation, the flow tube passes through a corresponding through hole, and then the flow tube and the vibrating sheet are welded fixedly along the entire circumference of the through hole by means of manual argon arc welding.
The U.S. Pat. No. 6,415,668 B1 published on Jul. 9, 2002 also provides a different type of vibrating sheet, as shown in FIG. 9. Such a vibrating sheet is mainly characterized in that the vibrating sheet consists of four parts welded together, two through holes for the passage of the flow tubes are respectively located in two separate half sheets, two connecting sheets connect the two half sheets together after the two flow tubes extend trough said two half sheets. Such a vibrating sheet is mainly adapted for the circumstance where the two flow tubes cannot extend through the two through holes in the vibrating sheet simultaneously. Thus, one through hole is arranged on each of the two separate half sheets, and then connected the two separate half sheets after the flow tubes extend through the half sheets. In this patent, each half sheet is fixedly welded to the flow tube along the entire circumference of the through hole.
No matter which vibrating sheet in the prior art, they all have a planar structure (namely, the welds formed and the main connecting portion of the vibrating sheet are substantially in the same plane or parallel planes) generally, and the prior art vibrating sheet and the flow tube are finally fixed by means of welding along the entire circumference of the through hole because those skilled in the art all believe that welding of the vibrating sheet and the flow tube along the entire circumference of the through hole can ensure a better connection strength.
In terms of the selection of the welding technique, the prior art usually adopts fusion welding or brazing to weld the vibrating sheet and the flow tube together.
However, for fusion welding such as argon arc welding, plasma welding or laser welding, the flow tube must be locally heated to a fusion state for achieving a better welding effect. During cooling and solidification process, metallic grains in the fused section of the flow tube will be rearranged, which may render coarse the grain structure in this section. Thus, the grain structure is very coarse along the entire circumference at the junction between the flow tube and the vibrating sheet, i.e., an annular weld 180 as shown in FIG. 10. The coarse grain structure will reduce the structural strength of the flow tube, which may result in flow tube breakage that often occurs in actual application.
Furthermore, the requisite local heating in the fusion welding will generate residual stress, which has a great influence on the precision of flow measurement. Upon welding the vibrating sheet by means of local heating, those skilled in the art can all appreciate that the residual stress will be generated in the weld region due to uneven heating of the material. The magnitude and direction of the residual stress will keep changing as time and vibration of the flow tube go on. The residual stress and the stress generated by vibration will be superposed. The magnitude and direction of the superposed stress will also change continuously, which will directly lead to inaccurate measurement of phase difference of the flow tube.
In persistent research of the technique for welding the vibrating sheet and the flow tube, those skilled in the art have made attempts for dozens of years, and would like to choose vacuum brazing technique to fix the vibrating sheet and the flow tube in most cases at the present because this technique causes minimum damages to the flow tube. For eliminating impact of the residual stress, many companies at the present tend to use a zinc-copper alloy or a nickel alloy as a brazing material to braze the entire flow tube in a vacuum environment. Since the brazing temperature can only melt the brazing material and is lower than the fusing temperature of the flow tube, there is no adverse influence on the flow tube performance. Uneven heating will never occur upon holistic heating of the flow tube, thereby producing no residual stress.
However, brazing has its own disadvantages. Since the brazing material is expected to have a fusing point different from that of the flow tube and the vibrating sheet, the brazing material must be different from that of the latter. Additionally, the brazing material often has hardness lower than that of the flow tube and the vibrating sheet, so the location of the brazing material is a weak part of the entire structure. With vibration of the flow tube, the brazing material is pressed and stretched constantly and will creep slowly, and the vibration state of the flow tube will also change gradually, thereby influencing the performance of flow measurement.
Moreover, the other main problem of brazing is that it must be conducted in a vacuum environment, and the weld should be void of bubbles and impurities. Once the weld has bubbles therein, the compressibility of the brazing material will be increased due to low hardness thereof. The weld will come to a failure very early during stretching and compression. It is very easy to braze a tube of relatively small size in a vacuum environment; however, there are numerous difficulties for a flow meter of larger size for the reason that the size of the vacuum furnace is not suitable for the large-sized flow meter or the vacuum degree cannot meet the requirement of vacuum brazing. As such, there are high requirements set for vacuum brazing, and subsequent rigorous examination is necessary for quality guarantee. Brazing is costly and technically difficult.